Gasoline and some other petroleum-derived hydrocarbons are among the most common contaminants of soil and groundwater. These compounds often mix with drinking water causing serious health concerns. Development of effective and economical remediation techniques is essential since groundwater is a source of drinking water in many areas of the world.
Known treatment techniques involve pump-and-treat methods, air stripping and steam flushing combined with vacuum extraction. Natural attenuation processes have been shown to reduce the concentration of contaminants in soil and groundwater, but these processes are usually slow and may take many years to decontaminate an affected area.
Recently, permeable under-ground walls and barriers have gained recognition as an alternative for the treatment of groundwater. Such techniques are more economical when implemented in-situ rather than ex-situ. The former also removes the need to transfer the affected material from the site thus eliminating the risk of phase transformation of volatile compounds. The underground biobarriers are passive reactors as they operate by natural hydraulic gradients of the underground stream, and they do not need pumping equipment to supply the contaminated stream to the reactor nor to pass the treated stream back underground. There is therefore a substantial difference between the in-situ passive underground biobarriers and above-ground ex-situ biological reactors for similar purposes (bioremediation of underground contaminated streams), the latter exemplified by U.S. Pat. No. 5,080,782 to Caplan.
Biological in-situ techniques and systems are particularly attractive compared with chemical ones since they have the potential to completely destroy the target contaminants leaving non-toxic chemicals as the products of biodegradation. The so-called in-situ bioremediation techniques have been successfully applied in the remediation of groundwater contaminated with petroleum hydrocarbons and chlorinated compounds.
Starr and Cherry (Ground Water, Vol. 32, NO. 3, May-June 1994) discuss various types of in-situ reactors and the packings used. Some reactors use a packing that modifies pH or Eh conditions in the subsurface. Others use a material (e.g. hydroxyapatite) that dissolves or causes precipitation of a mineral phase that immobilizes the contaminant. Another type of reactor removes contaminants mostly by sorption. Activated carbon and peat moss are the most commonly used materials in this category. Zeolites or synthetic ion exchange resins can also be used.
A number of patents pertain to various structures and features of the in-situ biological barriers for removal of organic contaminants. Exemplary in this regard are U.S. Pat. No. 5,057,221 to Bryant et al.; U.S. Pat. No. 5,384,048 to Hazen et al.; U.S. Pat. No. 5,624,552 to Vales et al.; U.S. Pat. No. 5,628,364 to Trenz; U.S. Pat. No. 5,518,620 to Eguchi et al.; and U.S. Pat. No. 5,389,248 to Pare et al. Bryant et al. deal with halogenated hydrocarbons and use activated carbon bed to support methylotrophic and heterotrophic microorganisms thereon. The specification mentions, without elaboration, that other substrates can also be utilized.